1. Field of the Invention
The present invention relates to a protective relay system for protecting the power system from line fault, and more specifically to a protective relay system which transmits electric information detected from each of the portions in the power system to a central control unit via a transmission line, and which transmits instruction from the control unit to each of the portions via the transmission line.
2. Description of the Prior Art
A conventional system for protecting the power system such as power transmission line, is illustrated in FIG. 1. Here, power transmission lines 1 must be protected from a line fault such as grounding, and provide a protective section via circuit breakers 2, 3, 4, 5 and 6. Electric currents flowing through the circuit breakers 2, 3, 4, 5 and 6 are detected by current transformers 7, 8, 9, 10 and 11, and are input to a protection circuit 22 via circuit terminal boards or conversion signal transmitters 12, 13, 14, 15, 16 and via receiving signal converters 17, 18, 19, 20 and 21, which convert input signals to suitable signal levels. Relying upon the signals from the converters 17 to 21, the protection circuit 22 discriminates a line fault such as by differential protection means. When it is discriminated that a line fault has occurred, the protection circuit 22 provides opening signals to each of its corresponding lines. The opening signals are input to res control circuits 24, 25, 26, 27 and 28 where each is amplified and is converted into an electromagnetic signal, to open the corresponding circuit breakers 2, 3, 4, 5, and 6.
According to the conventional protective relay system, as will be obvious from the above description, cables run along the transmission lines to transmit electric signals detected at a plurality of portions of the transmission lines to remote control circuits. Therefore, electrostatic or electromagentic noise is easily induced into the cables, giving rise to the occurrence of erroneous operation. Further, when the system is so designed that the control circuits detect differential currents flowing through the transmission lines, detectors must be provided to detect breakage of the cables in order to prevent erroneous operation in case any cable is broken. Accordingly, there are required adjustments in order to get conformity with operations of other portions.